TGR5 is a G-protein-coupled receptor protein, and an agonist or antagonist thereof is reported to be useful for the treatment of central nervous diseases, inflammatory diseases and the like (see WO01/77325, WO02/84286). However, a low molecular weight synthetic compound useful as a TGR5 agonist or antagonist has not been reported.
On the other hand, as a fused ring compound, the following compounds are reported.    1) A compound having a somatostatin receptor agonistic action, which is represented by the formula
wherein ring A is an aromatic hydrocarbon optionally having substituent(s) or an aromatic heterocycle optionally having substituent(s); ring B is an aromatic hydrocarbon optionally having substituent(s) or an aromatic heterocycle optionally having substituent(s); Z is a cyclic group optionally having substituent(s) or a chain hydrocarbon group optionally having substituent(s); R1 is a hydrogen atom, a hydrocarbon group optionally having substituent(s) or a heterocyclic group optionally having substituent(s); R2 is an optionally substituted amino group; D is a bond or a divalent hydrocarbon group optionally having substituent(s); E is a bond, —CON(Ra)—, —N(Ra)CO— or the like (wherein Ra is a hydrogen atom or a hydrocarbon group optionally having substituent(s)); G is a bond or a divalent hydrocarbon group optionally having substituent(s); L is a divalent group; ring B is optionally linked with R2 to form a non-aromatic fused nitrogen-containing heterocycle optionally having substituent(s); X is two hydrogen atoms, an oxygen atom or a sulfur atom; and Y is a nitrogen atom, an oxygen atom, —N(R4)— (wherein R4 is a hydrogen atom, a hydrocarbon optionally having substituent(s) or an acyl group) or S(O)n (wherein n is 0, 1 or 2), or a salt thereof (see WO98/47882).    2) A compound useful for the treatment of brain disorder, memory disorder and the like, which is represented by the formula
wherein R is H or the like; R1 is H; R2, R3 and R4 are each independently H, alkyl, or aralkyl wherein aromatic moiety is optionally substituted; or R1 and R2 in combination form a bond; R5 and R6 are each independently H, a halogen, a nitro, a cyano, a trifluoromethyl, a lower alkyl or a lower alkoxy; and n is an integer of 1 to 3, or a salt thereof (see U.S. Pat. No. 4,647,560).    3) A compound having an enzyme induction activity, which is represented by the formula
wherein R1 is a hydrogen, a halogen, a trifluoromethyl or a nitro group; R2 is a hydrogen atom or a C1-6 alkyl; R3 is an optionally substituted lower alkyl group or the like; R4 is a hydrogen, a chlorocarbonyl or a carbamoyl; and X is a hydrogen atom, a halogen or a trifluoromethyl, or a salt thereof (see U.S. Pat. No. 4,329,341).    4) A compound useful as a cholecystokinin (CCK) antagonist, which is represented by the formula
wherein R1 is H, a C1-5 straight chain or branched alkyl or the like; R2 is H, a lower alkyl, a substituted or unsubstituted phenyl or the like; R3 is —(CH2)n-R7 (wherein n is 0 to 4 and R7 is a α- or β-naphthyl or the like) or the like; R9 and R10 are each independently H, OH or CH3; R13 is H, O, a lower alkyl, an acyl or a cyclo lower alkyl; p is 0 or 1; X1 is H or the like; X7 is O, S or the like; and r is 1 or 2, or a salt thereof (see EP-167919A).    5) A compound of the following formula:
(see Bioorg. Med. Chem. Lett., 6: 267-272 (1996)).    6) A compound of the following formula:
(see Bioorg. Med. Chem., 7: 2427-2436 (1999)).    7) A compound of the following formula:
(see Croatica Chem. Acta, 62: 245-265 (1989)).    8) A compound of the following formula:
(see Chem. Pharm. Bull., 21: 742-751 (1973)).    9) A compound having a tachykinin antagonistic action and useful for inflammation, allergic disease and the like, which is represented by the formula
wherein ring M is a heterocycle having —N═C<, —CO—N< or —CS—N< as a partial structure: —X═Y<; Ra and Rb in combination form ring A, or are the same or different and each is independently a hydrogen atom or a substituent for ring M; ring A and ring B are each independently an optionally substituted homocycle or heterocycle, and at least one of them is an optionally substituted heterocycle; ring C is an optionally substituted homocycle or heterocycle; ring Z is an optionally substituted ring; and n is an integer of 1 to 6, or a salt thereof (see EP-733632A, WO99/47132).